1. Field of the Invention
The present invention relates to a spark plug for use in an internal combustion engine and, more particularly, to a spark plug having a sintered ceramic resistor assembled therein for preventing radio wave noise.
2. Description of the Related Art
A spark plug known in the related art for an internal combustion engine comprises: a cylindrical insulator having a through hole in an axial direction; a center electrode fitted in one end portion of the through hole; an external terminal fitted in the other end portion of the through hole; and a main fitting fitted on the outer circumference of the insulator. Moreover, gas tightness between the center electrode and the external terminal and the through hole of the insulator is maintained by a glass sealing method. In this method a conductive glass seal member substantially composed of a mixture of metal powder and glass powder is filled in the through hole between the center electrode and the external terminal, to thereby make an electric connection between the center electrode and the external terminal.
This engine spark plug generates interfering radio waves at the time of spark discharge which adversely affect various kinds of electronic devices. To address this problem, a spark plug having a resistor has been proposed, which is provided with both functions as spark plug and as a radio wave noise preventer. This resistor spark plug can be broadly classified into a monolithic type and a cartridge type resistor spark plug depending on the properties of the resistor.
The monolithic type spark plug is manufactured (as referred to in JP-A-51-27494, for example): by inserting the center electrode into the through hole of the insulator, filling the through hole on the rear end side of the center electrode with a conductive glass seal material powder of a mixture of glass powder and metal powder, a glass quality resistor composite powder of a mixture of ceramic powder, carbon black, a carbon substance and glass powder, and the conductive glass seal material powder in the recited order, and heating these fillers to a high temperature (e.g., 800° C. to 1,000° C.). The external terminal is thereby hot-pressed in the through hole of the insulator while the conductive glass seal material powder and the glass quality resistor composite powder are softened, so as to seal the space between the center electrode and the external terminal.
This monolithic type spark plug can be manufactured mainly by the step of filling the conductive glass seal material powder and the glass quality resistor composite powder in the through hole of the insulator and heating the filler. As such, this technique requires a small number of manufacturing steps, has excellent productivity and provides a durable product.
On the other hand, the cartridge type spark plug is manufactured: by inserting the center electrode into the through hole of the insulator; filling the conductive glass seal material of a mixture of the glass powder and the metal powder; inserting a coil resistor having an electric resistance material formed helically on the surface of the insulator; filling the conductive glass seal material; and heating those materials to a high temperature (e.g., 800° C. to 1,000° C.) to hot-press the external terminal in the through hole of the insulator and thereby seal the center electrode and the external terminal.
This coil resistor is exemplified by: one (as referred to in JP-A-49-116559, for example), in which a helical groove is formed in the surface of a column-shaped insulator and in which a resistive cover film is formed on the helical groove; by one (as referred to in JP-A-61-135079, for example), in which the column-shaped insulator is printed on its surface with a helical electric resistance material and is sintered; or by one (as referred to in JP-A-1-283784, for example), in which the cover film is made with a specific thickness to set its resistance and temperature dependency. Generally, the cartridge type spark plug using the coil resistor is superior in noise preventing effect as compared to the monolithic type because of less noise current.
3. Problems to be Solved by the Invention
Although the monolithic spark plug has excellent productivity and durability, it is difficult to make the resistor sufficiently long relative to the insulator through hole and to accordingly improve the noise preventing effect. This is because the manufacturing technique is restricted to filling the insulator with the conductive glass seal material powder and the glass quality resistor composite powder, and hot-pressing the external terminal in the through hole of the insulator.
The cartridge type spark plug using the coil resistor provides an excellent noise preventing effect but has insufficient durability. This is because a coil made by an electric resistance material is easily broken. Moreover, in the case that the conductive glass seal material powder is heated for the sealing operation so as to improve gas-tightness, the coil may not endure heating at the requisite high temperature. In order to heat and seal the conductive glass seal material powder, therefore, the use of a sealing terminal has been proposed. In the case of using this sealing terminal, however, the length of the sealing terminal makes it difficult to make the coil resistor sufficiently long relative to the insulator through hole, and accordingly it is difficult to improve the noise preventing effect.
Especially in recent years, increased use of a computer for complicated controls of an internal combustible engine has created a great demand for an effective noise prevention spark plug.